In recent years, study has been vigorously forwarded in the fields of nano-structures and nano-devices, and a spectrophotometric technology featuring a high resolution has been desired for evaluating properties of a variety of samples in these fields. In the silicon devices, for example, strain in Si seriously affects the device characteristics such as mobility and the like. Therefore, it is very important to know spatial distribution of the strain in Si device with a high resolution. One of the strain measurement methods is based on the Raman measurement. The Raman measurement is based on a principle that a peak position of a Raman signal shifts depending upon the strain. Upon mapping peak positions of Raman signals, therefore, it is allowed to know the distribution of strain.
The optical measurement with a high spatial resolution has heretofore been conducted by using a microscope. However, the above microscopic optical measurement encounters a barrier of diffraction limit which makes it difficult to accomplish the space resolution of finer than one micron. In modern silicon devices, the structural sizes are reaching the orders of submicrons and nanometers, and a measuring method of a higher resolution is desired. In recent years, therefore, various attempts have been made for improving the spatial resolution relying upon the near-field spectrophotometry by using a probe such as an optical fiber.
This method uses a near-field light leaking from a very small aperture at the end of the probe. Therefore, when it is attempted to observe maintaining a resolution of finer than 100 nm, the aperture size, too, must be decreased to be smaller than 100 nm, resulting in a very great loss of light quantity and arousing such a serious difficulty in the measurement that the method can be applied to only those samples that produce large signals. In the case of the Raman measurement of silicon, in particular, the optical fiber itself contains silicon which is a cause of disturbing the emission of Raman signals making it further difficult to take a measurement.
To solve this difficulty, one of the technologies proposed in the field of Raman spectroscopy uses a metallic AFM (atomic force microscope) probe. According to this method, Raman signals are enhanced only near the end of the probe due to a local electric field at an end of the metal probe, enhancing the space resolution. In this method, a large enhancing effect is obtained when two metals are brought close to each other maintaining a very small gap and when a sample to be measured is placed in the gap. Therefore, though the result can be obtained to some extent in the measurement of molecules and ultra-fine particles, the method cannot still be applied to the measurement of solid materials. This is because the sample to be measured which is a solid material cannot be placed between the two metals described above. Besides, strong signals in the far visual field are excited at positions away from the metal probe and conceal the signals in the near field.
The following patent document 1 discloses technology which uses a transmission type electron microscope to detect fine crystalline distortion in semiconductors. The image obtained by the transmission type electron microscope can be converted into a digital image, and the pattern can be calculated by two-dimensional Fourier transform.
[Patent document 1] JP-A-2000-65762